The sequence of tenses refers to the grammatical delineation of time in relation to a verb. Though time is referenced by auxiliary words or phrases modifying a verb, a verb’s tense tells a reader or listener the time-frame of an action — basically, whether the action was in the past, is in the present, or will be in the future; and whether the action is ongoing. Tenses usually seek agreement between clauses, as when a past tense in a subordinate clause is reflecting past tense usage in a main clause. As in all grammar usage, there are exceptions to the six basic rules for plain verb usage.
The six basic rules for simple usage of verbs cover present, past, and future tenses. Present tense expresses actions taking place right now, past tense expresses completed actions, and future tense expresses actions anticipated. Examples of these tenses, respectively, are: the old dog sleeps too much, the old dog slept too much, and the old dog will sleep too much. The remaining three of the six basic rules are for present perfect tense, past perfect tense, and future perfect tense. Example clauses for these tenses are: the old dog has slept too much; yesterday, Jackie noticed the old dog had slept too much; and the old dog will have slept too much before his supper time.
More explicit usage of a sequence of tenses can refer to continuous actions in present or past, called present progressive and past progressive. A present progressive action might be: I am talking on the phone right now. A past progressive might be: Yesterday, I was talking on the phone.
Present infinitive, past tense, and past participles are used with irregular verbs to show a relation to time. An example of present infinitive with the verb "sleep" is: The old dog wants to sleep too much. In simple past tense, the sentence might be: The old dog slept too much yesterday. An example of past participle usage is: After the old dog slept too much, he declined to eat his supper.